Gently, @RogueFemale have you ever had any counselling since the period of your sexual assaults? If you can afford it or access via your GP, this might be an ideal opportunity to try to access some CBT.
I understand that you feel the way you feel, but your projections in your own head of what happens once you've said hello back are doing some extremely heavy lifting here!
Yes the guy might have learning difficulties. Yes he clearly is physically enormous. That doesn't mean that a very quick 'hello' or 'morning' back from the opposite pavement means he's going to stalk you or see you as a target.
On the positive side, maybe it's a great thing to open a conversation with the neighbours in your new area though? "Ooh, how long has the guy at number 73 lived there? I thought it was a bit unusual he was still outside when I did my return trip from the shop as well."
Then you might get a range of useful responses anywhere between 'yes, he's a right wrong'un, on the paedophiles register, been done for stalking 3 times, definitely don't say hello he'll try to say something more back' through to the (probably statistically more likely) 'oh Bob, he's lived there all his life, been on his own since his Dad died 3 years ago. X person comes in Y times a week to look in on him, but he never moves off his doorstep for anything. Completely harmless, say hello & he never say anything more that that.' Or maybe somewhere in between.
I do understand your concerns about the "Of Mice & Men" character, and obviously you bring your own history with you to assessing potential risks as statistics haven't been in your favour in the past. Talk to your (other!) new neighbours, find out some more background, but don't treat the 90+% of us who would simply say hello back & keep walking like we're the outliers.
Hello doesn't have to lead to anything, it doesn't have to mean anything, it isn't consent & it wouldn't usually lead to stalking or even necessarily another sentence.
Maybe try to access some CBT through your doctor to help you parse the world in a way that helps you live your life in a lower state of fight or flight? I understand how you've got there, and I'm not telling you that's wrong - but you could probably access a far more fulfilling & enjoyable life from now on if you can get some help with living in a lower stress state as you move through the world.
Good luck 